The invention relates to extruded alkenyl aromatic polymer foams comprising a blend of 1,1,2,2-tetrafluoroethane (HFC 134) and a low solubility co-blowing agent and a process for making.
Due to increasingly stringent environmental regulations, manufacturers of alkenyl aromatic polymer insulating foams are considering fluorocarbons as potential insulating cell gas agents (blowing agents). One such agent is HFC 134.
HFC 134 is known in the art as a blowing agent for making alkenyl aromatic polymer foams. HFC 134 is disclosed in Canadian Patent 1,086,450; Japanese Published Application 5-287111; and U.S. Pat. Nos. 5,278,196 and 5,182,040.
When selecting a blowing agent for insulating alkenyl aromatic polymer foams, the amount of insulating blowing agent must be high enough to provide adequate insulating performance but not high enough to result in undesirable physical properties in the foam product.
A problem with using HFC 134 as a blowing agent is that at levels required to maintain desirable foam insulating performance (i.e. about 70 weight percent or more based upon the total moles of blowing agent), poor dimensional stability is observed when producing closed cell alkenyl aromatic polymer foams of low density (i.e. about 16 to about 64 kilograms per cubic meter) and thick cross-section (i.e. about 15 millimeters or more).
The prior art does not effectively address the dimensional stability problem associated with use of high levels of HFC 134 in blowing agents when making closed cell alkenyl aromatic polymer foams of low density and thick cross-section.
Canadian Patent No. 1,086,450 relates extruded alkenyl aromatic polymer foams made with 30 to 70 weight percent of a low permeability blowing agent such as HFC 134 and a high permeability co-blowing agent such as flourochloromethane, methyl chloride, ethyl chloride, chlorodifluoromethane, and 1,1-difluoroethane. The Canadian patent avoids the above problem when it employs significantly lower levels of HFC 134. The disclosed co-blowing agents are very soluble in alkenyl aromatic polymers and negatively impact foam dimensional stability when employed with HFC 134 in blowing agent compositions comprising about 70 weight percent or more of HFC 134.
Japan Published Application 5-287111 relates extruded polystyrene foams made with a blowing agent of HFC 134 and 2-chloro-1,1,1,2-tetraflouroethane (HCFC 124) in an 80/20 ratio by weight. Although the dimensional stability of the disclosed foams is not described, use of such blowing agents would result in dimensionally unstable foams due to the high solubility of HCFC 124 in polystyrene and its slow permeation rate through polystyrene foams.
U.S. Pat. No. 5,182,040 relates azeotropic and near-azeotropic compositions described as useful as blowing agents in polymer foams among other uses. The compositions comprise HFC 134 and a co-blowing agent selected from among 1,1-difluoroethene (HFC 152a); 1-chloro-1,1-difluoroethane (HCFC 142b); dimethyl ether (DME); 1,1,1,2,3,3,3,-heptafluoropropane (HFC 227ea); perfluorocyclobutane (HFC 318); n-butane; or isobutane. A specific foam embodiment of a polystyrene foam blown with a composition of 60-99 weight percent HFC 134 and 1-40 weight percent DME of certain physical properties (claim 1). Although the dimensional stability of any possible foams is not described, use of such co-blowing agents with about 70 weight percent or more HFC 134 would result in dimensionally unstable foams due to their high solubility in polystyrene and, in the case of some of the co-blowing agents, their slow permeation rate through polystyrene foams. Further, DME is very flammable.
U.S. Pat. No. 5,278,196 relates plastic foams blown with a composition of 1,1,1,2,3,3,3-heptafluoropropane (HFC 227ea) and HFC 134 (claim 7). The reference does not describe a dimensionally stable, low density, extruded polystyrene foam blown with a composition comprising about 70 weight percent or more of HFC 134.
It was found surprising that a dimensionally stable, low density, extruded alkenyl aromatic polymer foam of low density and thick cross-section could be produced with a blowing agent formulation comprising about 70 weight percent or more of HFC 134a.